Patent application title: Repetition coding for a wireless system

Abstract:

A system and method are disclosed for transmitting data over a wireless
channel. In some embodiments, transmitting data includes receiving
convolutionally encoded data and enhancing the transmission of the data
by further repetition encoding the data.

Claims:

1. (canceled)

2. A receiver, comprising:an antenna configured to receive a signal
transmitted on a wireless channel;a mask remover configured to remove a
mask to obtain a de-masked signal;a combiner configured to combine data
associated with the de-masked signal to obtain a combined signal,
including by:obtaining a first set of data associated with a first
subchannel;obtaining a second set of data associated with a second
subchannel;determining a first weight based at least in part on a first
measure of quality associated with the first subchannel;determining a
second weight based at least in part on a second measure of quality
associated with the second subchannel; andcombining the first set of data
and the second set of data based at least in part on the first weight and
the second weight; anda decoder configured to decode a signal associated
with the combined signal.

3. The receiver of claim 2 further comprising a Fast Fourier Transform
(FFT) configured to convert a time domain signal into a frequency domain
signal, wherein the frequency domain signal is passed to the mask
remover.

4. The receiver of claim 2 further comprising a de-interleaver, wherein
de-interleaving processing is performed before combining processing
associated with the combiner.

5. The receiver of claim 4, wherein the de-interleaver is configured to
receive the combined signal from the combiner and/or passes the
de-interleaved signal to the decoder.

6. The receiver of claim 2 further comprising a de-interleaver, wherein
de-interleaved processing is performed after combining processing
associated with the combiner.

7. The receiver of claim 6, wherein the de-interleaver is configured to
receive the de-masked signal from the mask remover and/or passes the
de-interleaved signal to the combiner.

8. The receiver of claim 2, wherein the decoder includes a Viterbi
decoder.

9. The receiver of claim 2, wherein:the signal received by the antenna is
transmitted by a transmitting device; andthe transmitting device includes
a masking module configured to apply a mask such that the peak to average
ratio of the signal output by the masking module is less than the peak to
average ration of the signal input by the masking module.

10. The receiver of claim 9, wherein the mask applied includes a
pseudorandom value.

11. A method for processing a received signal, comprising:receiving a
signal transmitted on a wireless channel;removing a mask to obtain a
de-masked signal;combining data associated with the de-masked signal to
obtain a combined signal, including by:obtaining a first set of data
associated with a first subchannel;obtaining a second set of data
associated with a second subchannel;determining a first weight based at
least in part on a first measure of quality associated with the first
subchannel;determining a second weight based at least in part on a second
measure of quality associated with the second subchannel; andcombining
the first set of data and the second set of data based at least in part
on the first weight and the second weight; anddecoding a signal
associated with the combined signal.

12. The method of claim 11 further comprising performing Fast Fourier
Transform (FFT) processing to convert a time domain signal into a
frequency domain signal, wherein the frequency domain signal is passed to
the mask remover.

13. The method of claim 11 further comprising de-interleaving, wherein the
de-interleaving processing is performed before combining.

14. The method of claim 11 further comprising de-interleaving, wherein the
de-interleaving processing is performed after combining.

15. The method of claim 11, wherein:the received signal is transmitted by
a transmitting device; andthe transmitting device is configured to
perform masking where a mask is applied such that the peak to average
ratio of the output is less than the peak to average ration of the input.

16. A computer program product for processing a received signal, the
computer program product being embodied in a computer readable storage
medium and comprising computer instructions for:receiving a signal
transmitted on a wireless channel;removing a mask to obtain a de-masked
signal;combining data associated with the de-masked signal to obtain a
combined signal, including by:obtaining a first set of data associated
with a first subchannel;obtaining a second set of data associated with a
second subchannel;determining a first weight based at least in part on a
first measure of quality associated with the first subchannel;determining
a second weight based at least in part on a second measure of quality
associated with the second subchannel; andcombining the first set of data
and the second set of data based at least in part on the first weight and
the second weight; anddecoding a signal associated with the combined
signal.

17. The computer program product of claim 16 further comprising computer
instructions for performing Fast Fourier Transform (FFT) processing to
convert a time domain signal into a frequency domain signal, wherein the
frequency domain signal is passed to the mask remover.

18. The computer program product of claim 16 further comprising computer
instructions for de-interleaving, wherein the de-interleaving processing
is performed before combining.

19. The computer program product of claim 16 further comprising computer
instructions for de-interleaving, wherein the de-interleaving processing
is performed after combining.

Description:

CROSS REFERENCE TO OTHER APPLICATIONS

[0001]This application is a continuation of co-pending U.S. patent
application Ser. No. 10/666,952 entitled REPETITION CODING FOR A WIRELESS
SYSTEM filed Sep. 17, 2003 which is incorporated herein by reference for
all purposes.

FIELD OF THE INVENTION

[0002]The present invention relates generally to a data transmission
scheme for a wireless communication system. More specifically, a
repetition coding scheme for a wireless system is disclosed.

BACKGROUND OF THE INVENTION

[0003]The IEEE 802.11a, 802.11b, and 802.11g standards, which are hereby
incorporated by reference, specify wireless communications systems in
bands at 2.4 GHz and 5 GHz. The combination of the 802.11a and 802.11g
standards, written as the 802.11a/g standard, will be referred to
repeatedly herein for the purpose of example. It should be noted that the
techniques described are also applicable to the 802.11b standard where
appropriate. It would be useful if alternate systems could be developed
for communication over an extended range or in noisy environments. Such
communication is collectively referred to herein as extended range
communication. The IEEE 802.11a/g standard specifies a robust data
encoding scheme that includes error correction. However, for extended
range communication, a more robust data transmission scheme at reduced
data rates is required.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]The present invention will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural elements, and
in which:

[0005]FIG. 1A is a diagram illustrating the data portion of a regular
802.11a/g OFDM packet.

[0006]FIG. 1B is a diagram illustrating the data portion of a modified
802.11a/g OFDM packet where each symbol is repeated twice (r=2).

[0007]FIG. 2A is a diagram illustrating a transmitter system with a
repetition encoder placed after the output of an interleaver such as the
one specified in the IEEE 802.11a/g specification.

[0008]FIG. 2B is a diagram illustrating a receiver system for receiving a
signal transmitted by the transmitter system depicted in FIG. 2A.

[0009]FIG. 3A is a diagram illustrating a transmitter system with a
repetition encoder placed before the input of an interleaver designed to
handle repetition coded bits such as the one described below

[0010]FIG. 3B is a diagram illustrating a receiver system for receiving a
signal transmitted by the transmitter system depicted in FIG. 3A.

[0011]FIGS. 4A-4C are tables illustrating an interleaver.

DETAILED DESCRIPTION

[0012]It should be appreciated that the present invention can be
implemented in numerous ways, including as a process, an apparatus, a
system, or a computer readable medium such as a computer readable storage
medium or a computer network wherein program instructions are sent over
optical or electronic communication links. It should be noted that the
order of the steps of disclosed processes may be altered within the scope
of the invention.

[0013]A detailed description of one or more preferred embodiments of the
invention is provided below along with accompanying figures that
illustrate by way of example the principles of the invention. While the
invention is described in connection with such embodiments, it should be
understood that the invention is not limited to any embodiment. On the
contrary, the scope of the invention is limited only by the appended
claims and the invention encompasses numerous alternatives, modifications
and equivalents. For the purpose of example, numerous specific details
are set forth in the following description in order to provide a thorough
understanding of the present invention. The present invention may be
practiced according to the claims without some or all of these specific
details. For the purpose of clarity, technical material that is known in
the technical fields related to the invention has not been described in
detail so that the present invention is not unnecessarily obscured.

[0014]In a typical system as described below, bits representing a set of
data that is to be communicated are convolutionally encoded or otherwise
transformed into values. Various types of modulation may be used such as
BPSK, QPSK, 16QAM or 32QAM. In the case of BPSK, which is described
further herein, each BPSK symbol may have one of two values and each BPSK
symbol corresponds to one bit. An OFDM symbol includes 48 values that are
transmitted on different subchannels. To provide extended range, each
value that is sent is repeated several times by the transmitter. In one
embodiment, the bits are convolutionally encoded using the same encoding
scheme as the encoding scheme specified for the IEEE 802.11a/g standard.
Each encoded value is repeated and transmitted. Preferably, the values
are repeated in the frequency domain, but the values may also be repeated
in the time domain. In some embodiments, the repetition coding is
implemented before interleaving and a specially designed interleaver is
used to handle repeated values. In addition, a pseudorandom code may be
superimposed on the OFDM symbols to lower the peak to average ratio of
the transmitted signal.

[0015]The receiver combines each of the signals that correspond to the
repetition coded values and then uses the combined signal to recover the
values. In embodiments where the values are combined in the frequency
domain, the signals are combined coherently with correction made for
different subchannel transfer functions and phase shift errors. For the
purpose of this description and the claims, "coherently" combining should
not be interpreted to mean that the signals are perfectly coherently
combined, but only that some phase correction is implemented. The signals
from different subchannels are weighted according to the quality of each
subchannel. A combined subchannel weighting is provided to a Viterbi
detector to facilitate the determination of the most likely transmitted
sequence.

[0016]Using the modulation and encoding scheme incorporated in the IEEE
802.11a/g standard, the required signal to noise ratio decreases linearly
with data rate assuming the same modulation technique and base code rate
are not changed and repetition coding is used. Some further gains could
be achieved through the use of a better code or outer code. However, in a
dual mode system that is capable of implementing both the IEEE 802.11a/g
standard and an extended range mode, the complexity introduced by those
techniques may not be worth the limited gains that could be achieved.
Implementing repetition of values is in comparison simpler and more
efficient in many cases.

[0017]The repetition code can be implemented either in the time domain or
in the frequency domain. For time domain repetition, the OFDM symbols in
the time domain (after the IFFT operation) are repeated a desired number
of times, depending on the data rate. This scheme has an advantage in
efficiency since just one guard interval is required for r-repeated OFDM
symbols in the time domain.

[0018]FIG. 1A is a diagram illustrating the data portion of a regular
802.11a/g OFDM packet. Each OFDM symbol 102 is separated by a guard band
104. FIG. 1B is a diagram illustrating the data portion of a modified
802.11a/g OFDM packet where each symbol is repeated twice (r=2). Each set
of repeated symbols 112 is separated by a single guard band 104. There is
no need for a guard band between the repeated symbols.

[0019]The OFDM symbols can also be repeated in the frequency domain
(before the IFFT). The disadvantage of this scheme is that one guard
interval has to be inserted between every OFDM symbol in the time-domain
since the OFDM symbols with frequency-domain repetition are not periodic.
However, repetition in the frequency domain can achieve better multipath
performance if the repetition pattern is configured in the
frequency-domain to achieve frequency diversity.

[0020]In a typical environment where signals are reflected one or more
times between the transmitter and the receiver, it is possible that
certain reflections and direct signals will tend to cancel out at the
receiver because the phase difference between the paths could be close to
180 degrees. For different frequencies, the phase difference between the
paths will be different and so spreading the repeated values among
different frequencies to achieve frequency diversity ensures that at
least some of the values will arrive at the receiver with sufficient
signal strength to be combined and read. To maximize the benefit of
frequency diversity, it is preferable to repeat values across subchannels
that are as widely spaced as is practicable, since the phase difference
between adjacent subchannels is small.

[0021]FIG. 2A is a diagram illustrating a transmitter system with a
repetition encoder placed after the output of an interleaver such as the
one specified in the IEEE 802.11a/g specification. In this example
system, BPSK modulation is implemented and the repetition encoder and the
interleaver are described as operating on bits, which is equivalent to
operating on the corresponding values. In other embodiments, other
modulation schemes may be used and values may be repeated and
interleaved. The interleaver is included in the IEEE 802.11a/g
transmitter specification for the purpose of changing the order of the
bits sent to remove correlation among consecutive bits introduced by the
convolutional encoder. Incoming data is convolutionally encoded by
convolutional encoder 202. The output of convolutional encoder 202 is
interleaved by IEEE 802.11a/g interleaver 204. Repetition encoder 206
repeats the bits and pseudorandom mask combiner 208 combines the output
of repetition encoder 206 with a pseudorandom mask for the purpose of
reducing the peak to average ratio of the signal, as is described below.
The signal is then processed by IFFT processor 210 before being
transmitted.

[0022]FIG. 2B is a diagram illustrating a receiver system for receiving a
signal transmitted by the transmitter system depicted in FIG. 2A. The
received signal is processed by FFT processor 220. The output of FFT
processor 220 is input to mask remover 218 which removes the pseudorandom
mask. Data combiner 216 combines the repetition encoded data into a
stream of nonrepetitive data. The operation of data combiner 216 is
described in further detail below. IEEE 802.11a/g deinterleaver 214
deinterleaves the data and Viterbi decoder 212 determines the most likely
sequence of data that was input to the transmission system originally.

[0023]The system depicted in FIGS. 2A and 2B can use the same interleaver
and deinterleaver as the regular 802.11a/g system, and also has
flexibility in designing the repetition pattern since the repetition
coder is placed right before the IFFT block. However, it has certain
disadvantages. Data padding is required at the transmitter and data
buffering is required at the receiver. Bits have to be padded according
to the number of bytes to be sent and the data rate. The number of padded
bits is determined by how many bits one OFDM symbol can carry. Since the
802.11a/g interleaver works with 48 coded bits for BPSK modulation, bits
need to be padded to make the number of coded bits a multiple of 48.
Since the repetition coder is placed after the interleaver, it may be
necessary to pad the data by adding unnecessary bits for lower data rates
than 6 Mbps.

[0024]For example, one OFDM symbol would carry exactly 1 uncoded repeated
bit at a data rate of 1/4 Mbps. Since the OFDM symbol could be generated
from that one bit, there would never be a need to add extra uncoded bits
and so padding would not be necessary in principle. However, due to the
special structure of the 802.11a/g interleaver, several bits would need
to be padded to make the number of coded bits a multiple of 48 before the
interleaver. The padded bits convey no information and add to the
overhead of the transmission, making it more inefficient.

[0025]On the other hand, if the repetition encoder is placed after the
interleaver, the repetition coded bits generated from the 48 interleaved
bits are distributed over multiple OFDM symbols. Therefore, the receiver
would need to process the multiple OFDM symbols before deinterleaving the
data could be performed. Therefore, additional buffers would be necessary
to store frequency-domain data.

[0026]The system can be improved and the need for data padding at the
transmitter and data buffering at the receiver can be eliminated by
redesigning the interleaver so that it operates on bits output from the
repetition encoder.

[0027]FIG. 3A is a diagram illustrating a transmitter system with a
repetition encoder placed before the input of an interleaver designed to
handle repetition coded bits such as the one described below. Incoming
data is convolutionally encoded by convolutional encoder 302. The output
of convolutional encoder 302 is repetition coded by repetition encoder
304. Interleaver 306 interleaves the repetition coded bits. Interleaver
306 is designed so that data padding is not required and so that for
lower repetition levels, the bits are interleaved so as to separate
repeated bits. Pseudorandom mask combiner 308 combines the output of
Interleaver 306 with a pseudorandom mask for the purpose of reducing the
peak to average ratio of the signal, as is described below. The signal is
then processed by IFFT processor 310 before being transmitted.

[0028]FIG. 3B is a diagram illustrating a receiver system for receiving a
signal transmitted by the transmitter system depicted in FIG. 3A. The
received signal is processed by FFT processor 320. The output of FFT
processor 320 is input to mask remover 318 which removes the pseudorandom
mask. Deinterleaver 316 deinterleaves the data. Data combiner 314
combines the repetition encoded data into a stream of nonrepetitive data.
The operation of data combiner 314 is described in further detail below.
Viterbi decoder 312 determines the most likely sequence of data that was
input to the transmission system originally.

[0029]Interleaver 306 is preferably designed such that the same (repeated)
data are transmitted well separated in the frequency domain to achieve
full frequency diversity. For example, a repetition pattern in the
frequency domain for in 1 Mbps mode in one embodiment would repeat each
bit 6 times. Denoting data in the frequency domain as d1, d2, .
. . , d8, the repeated sequence of data is given by:

[0030]d1d1d1d1d1d1d2d2d2d.sub-
.2d2d2 . . . d8d8d8d8d8d8

[0031]The same data are placed in a group fashion because it is easy to
combine those data at the receiver. Note that the repeated data can be
combined only after r (6 in this example) data are available.

[0032]The repetition pattern in the above example does not provide the
greatest possible frequency diversity since the spacing between the same
data transmitted on adjacent subchannels may not be large enough and the
subchannels corresponding to the same data are not completely
independent. Greater frequency diversity would be desirable especially
for multipath channels with large delay spreads. Interleaver 306,
therefore, is designed to spread the repeated data in the frequency
domain to achieve frequency diversity as much as is practical.

[0033]In one embodiment, the interleaver is designed to optimize the
frequency diversity provided by the interleaver for data rates faster
than 1 Mbps (repetition number <=6). For lower data rates 1/2 and 1/4
Mbps, there is enough repetition that sufficient subchannels are covered
to provide frequency diversity even if adjacent subchannels are used. In
the preferred interleaver described below, repeated bits are separated at
least by 8 subchannels and consecutive coded bits from the convolutional
encoder are separated at least by 3 subchannels. The interleaver is
designed according to the following steps:

[0034]1. A 6×8 table is generated as shown in FIG. 4A to satisfy the
first rule which specifies that bits are separated at least by 8
subchannels. [0035]2. As shown in FIG. 4B, the columns are swapped to
meet the second rule which specifies that consecutive coded bits are
separated at least by 3 subchannels. [0036]3. As shown in FIG. 4c,
separation between repeated bits is increased by swapping rows. In the
example shown, repeated bits are separated by at least 16 bins for 3 Mbps
(Repetition number=2 for 3 Mbps so each bit is repeated once.)

[0038]Repetition of the values in the frequency domain tends to generate a
peak in the time domain, especially for very low data rates (i.e., for
large repetition numbers). The large peak-to-average ratio (PAR) causes
problems for the system, especially the transmit power amplifier. This
problem can be ameliorated by scrambling or masking the values
transmitted on different frequencies so that they are not all the same.
As long as the masking scheme is known, the scrambling can be undone at
the receiver. In one embodiment, the frequency-domain data is multiplied
by the long symbol of 802.11a/g, which was carefully designed in terms of
PAR. As can be seen in FIG. 2, the mask operation is performed right
before the IFFT operation. In general, any masking sequence can be used
that causes repeated values to differ enough that the PAR is suitably
reduced. For example, a pseudorandom code is used in some embodiments.

[0039]At the receiver, decoding includes: (1) mask removal, (2)
deinterleaving, (3) data combining, (4) channel correction, (5) Viterbi
decoding. It should be noted that in some embodiments, the order of the
steps may be changed as is appropriate.

[0040]In embodiments using frequency repetition, the transmitter
preferably masks the frequency-domain signal to reduce the
peak-to-average ratio (PAR) in the time-domain. The receiver removes the
mask imposed by the transmitter. If, as in the example above, the mask
used by the transmitter consists of +/-1 s, then the mask is removed by
changing the signs of the FFT outputs in the receiver. After the mask is
removed, the data is deinterleaved according to the interleaving pattern
at the transmitter.

[0041]The repeated signal is combined in the frequency domain at the
receiver to increase the SNR of the repeated signal over the SNR had the
signal not been repeated. The SNR is increased by multiplying the complex
conjugate of the channel response as follows.

.di-elect cons. ##EQU00001## .di-elect cons. ##EQU00001.2##

[0042]where Yj is the signal in subchannel j; Hj is the response
of subchannel j, Yc is the combined signal, Hc is the combined
channel, and Sc is the set of indices corresponding to the frequency
subchannels that contain the same data.

[0043]The channel effect is preferably removed before the data is input to
the Viterbi decoder so that the Viterbi decoder is able to use the same
soft decision unit regardless of the actual channel response. In the
extended-range mode, the combined channel is used in the channel
correction unit.

[0044]The frequency-domain signals are weighted for calculating the
path-metrics in the soft-decision Viterbi decoder, and the optimal
weights are determined by the corresponding SNR.

[0045]The resulting SNR for the combined signal becomes:

.di-elect cons. σ ##EQU00002##

[0046]where Ex is the signal power, and σj2 is the
noise power for the subchannel j. The combined SNR is used to evaluate
the Viterbi weights.

[0047]The 802.11a/g standard specifies that there are four pilot signals
included in each OFDM symbol for the purpose of estimating timing offset
and frequency offset and tracking phase noise in 802.11a/g signals. The
802.11a/g system assumes that these 4 pilots are reliable enough to
estimate the phase information. That assumption may not be true for a
system with a very low SNR. The redundancy that exists in the
frequency-domain signal is exploited to help the pilots to estimate and
track phase.

[0048]The phase information is estimated from the frequency domain data as
follows:

[0049]1. The repeated signals are combined in the frequency domain to
increase the SNR, with a channel estimate determined from a preamble
sequence of long symbols and an estimated slope, which captures the
effect of timing offset.

[0050]2. Hard decisions are made for each of the combined signals after
removing the phase offset estimated from the previous symbol.

[0051]3. The combined signals are multiplied by their own hard decisions.
The average of the hard-decision corrected signal is used to evaluate an
angle to estimate the phase offset for the current symbol.

[0052]A filter is applied to the estimated phase offset to reduce the
effect of noise. In one embodiment, a nonlinear median filter is used.
The nonlinear median filter effectively detects and corrects an abrupt
change in the phase offset, which could be caused by hard decision
errors.

[0053]An encoding and decoding scheme for a wireless system has been
disclosed. Preferably, repetition coding in the frequency domain is used.
An interleaver that provides frequency diversity has been described. In
various embodiments, the described techniques may be combined or used
separately according to specific system requirements.

[0054]Although the foregoing invention has been described in some detail
for purposes of clarity of understanding, it will be apparent that
certain changes and modifications may be practiced within the scope of
the appended claims. It should be noted that there are many alternative
ways of implementing both the process and apparatus of the present
invention. Accordingly, the present embodiments are to be considered as
illustrative and not restrictive, and the invention is not to be limited
to the details given herein, but may be modified within the scope and
equivalents of the appended claims.